1. Field of the Invention
The present invention relates to optical disc drives, and more particularly, to a method of and an apparatus for controlling a power of a laser diode in an optical disc drive to stabilize a servo control operation by efficiently compensating for a slow tail effect occurring in photodiode monitoring outputs of the laser diode.
2. Description of Related Art
Data is recorded on an optical disc by forming marks/spaces on tracks of the optical disc. In a read-only disc such as a CD-ROM and a DVD-ROM, a mark is formed as a pit. In a recordable disc such as a CD-R/RW and DVD-R/RW/RAM, a mark is formed by irradiating a laser signal on a phase-change-type recording film (RW series), which is converted into one of a crystalline state and an amorphous state according to a temperature, or an organic color coating recording film (R and RAM series) in which optical transmittance is varied according to a temperature.
Until now, the data processing speed of an optical disc drive has been increased by increasing the rotation speed of the disc. Generally, the rotation speed of a conventional disc drive is 60 times faster than the normal reproduction speed. However, as the rotation speed of the optical disc drive increases, a higher recording frequency is required. And, as the recording frequency increases, servo control becomes more unstable.
Another important cause that severely affects the servo control is a slow tail effect due to a slew rate characteristic of a photodiode monitoring an output of a laser diode.
The slew rate is an index representing how fast an output signal replies to an input signal in the photodiode or an operational amplifier, and is computed as slew rate=output voltage variation (ΔV)/time variation (Δt) [V/μS].
The slew rate characteristic of the photodiode is inevitable and occurs due to carrier diffusion. The slew rate characteristic causes the slow tail effect due to which the output signal, i.e., an output of the photodiode, is not perfectly proportional to the input signal, i.e., an output of the laser diode, during a specified period of time. The faster the recording speed, the greater the influence of the slow tail effect.
Because of the slow tail effect, the output of the laser diode, which is sampled by an apparatus controlling the laser diode, becomes unstable. That is, since the photodiode cannot smoothly reflect the output of the laser diode when the slow tail effect occurs, the output level of the laser diode, as well as a read level of the laser signal, becomes unstable.
In general, since a servo system of the optical disc drive is designed in consideration of the read level of the laser signal, if the read level of the laser signal becomes unstable, a servo control operation becomes unstable too.
Experimental results regarding the slow tail effect show that the servo control operation becomes unstable at velocities higher than a 16× velocity, and a servo control signal level decreases by more than 30% at a 48× velocity.
To remove or reduce the effects of the slow tail effect, in a conventional method, the read level of the laser signal is compensated with a specified value in a section where the slow tail effect occurs, or sampling in a read section is performed at the end of the read section if possible.
However, the method of compensating the read level of the laser signal with the specified value cannot exactly reflect the influence of the slow tail effect. Also, since deviations according to different devices and deviations according to using conditions and used period of times even in the same device are not considered, reliability is low. In the method of controlling the sampling, since a faster recording speed requires a more accurate sampling control, the devices for performing the method are expensive, and even though the sampling is accurately controlled, a residual error still remains.